Humans perceive direction of sound to be coming from the position from where the first sound arrives.
If a sound from a second source (e.g. the delay speakers) arrives within 5 ms after the first source, then this second source also has an influence on the perceived direction of the sound and the listener will perceive the sound to be coming from a position somewhere between the delay speakers and the front PA.
If, on the other hand, a sound from the second source arrives between 5 ms and 35 ms after the first source, then this second source is perceived by the listener as a single reflection of the sound from the front PA and has no influence on the perceived direction of the sound - the overall sound is thus perceived to be coming from the direction of the front PA.
Finally, if a sound from the second source arrives more than 35 ms after the first source, then this second source is perceived by the listener as a separate sound source (an echo) and the listener perceives two separate directions of sound - from the front PA and from the delay speakers.
Therefore, giving the delay speakers between 5 ms and 35 ms (as a rule, set it to 15 ms) of delay allows you to bring more sound level to the back of the room without altering the perception of direction of sound.
This somewhat remarkable phenomena is known as the Haas Effect.
For example, the following sound track is delayed in the left channel by 3ms. Although the sound pressure level is the same in both channels, the sound is perceived as coming from the right-hand direction.